1. Technical Field
This invention relates to an air brake safety device for truck trailers. The invention also relates to a truck trailer equipped with the safety device.
2. Background Art
Air brake safety devices are used in truck trailers to protect workers loading and unloading the trailers. The devices prevent compressed air from a tractor from unlocking the air brakes on the trailer to allow the trailer to be driven away by the tractor while the workers are still working the trailer. The safety devices usually employ two-way valves which, in one position, allow air from the compressor on the tractor to reach the brakes on the trailer to unlock the brakes and allow the trailer to be moved. In a second position, the device prevents all the air from the compressor from reaching the brakes and instead diverts some of the air from the compressor to the atmosphere. In this position the trailer cannot be moved. Usually the air is diverted through an alarm such as a horn or whistle to the atmosphere to alert people working in the trailer that an attempt is being made to move it. The alarm can also be used as an anti-theft device to alert people that an attempt is being made to unlock the brakes in an attempt to steal the trailer when it is loaded and parked. Air from the compressor is still passed to the brakes but since most of the air is diverted to the atmosphere, the brakes cannot be unlocked. Examples of such safety devices are shown in U.S. Pat. Nos. 5,145,240 and 5,747,886 by way of example.
These known safety devices have some disadvantages however. Normally, the devices allow some air to reach the trailer brakes even while diverting most of the air to the atmosphere. If the outlet to the atmosphere, for any reason becomes blocked, the air pressure to the brakes may become sufficient to operate the brakes thus either endangering the people working at the back of the trailer, or allowing the trailer to be stolen. Some of the devices also continually operate an alarm while diverting most of the air to the atmosphere which can be annoying to the workers. The known safety devices are also usually quite complicated in construction and thus relatively expensive to build and install. The devices can also be somewhat difficult to operate depending on the location of the valve.